Valvular Heart Disease USMLE Step 1 Practice Question

A 42-year-old male with a history of intravenous heroin use presents to the emergency department with a 2-week history of fever, night sweats, and progressive dyspnea. On examination, he has a temperature of 38.9°C, a new holosystolic murmur best heard at the left lower sternal border that increases with inspiration, splenomegaly, and splinter hemorrhages under his fingernails. Blood cultures grow methicillin-sensitive Staphylococcus aureus. Transthoracic echocardiography demonstrates large vegetations with severe regurgitation involving one of the cardiac valves. Which of the following valves is most commonly affected in this clinical presentation?